The major goal of the proposed research is to elucidate central molecular controls and regulatory mechanisms critical for development, identity, and subtype diversity of neocortical corticofugal (output) projection neurons (CFuPN), which might also enable their future directed differentiation, or molecular-cellular manipulation / ?reprogramming?. CFuPN are the broad population of cerebral cortex excitatory output neurons that extend axons away from cortex to subcortical targets. Dysregulation of CFuPN developmental controls contributes to human developmental diseases? and potentially later to selective vulnerability in neurodegenerative diseases. CFuPN include corticospinal motor neurons (CSMN), the broader subpopulation of subcerebral projection neurons (SCPN), corticothalamic projection neurons (CThPN), and major subsets of corticostriatal projection neurons (CStrPN). They play key roles in motor, sensory, high-level cognitive, and behavioral functions. Distinct CFuPN subtypes are specifically vulnerable and central to distinct disorders. CSMN/SCPN degenerate with spinal motor neurons in ALS; CStrPN with striatal MSN in Huntington's disease. CSMN damage is central in spinal cord injury. Cerebral palsy involves CFuPN injuries; some pain syndromes and epilepsies, CThPN. Understanding controls over development and maintenance of CFuPN diversity and precision will contribute centrally to basic understanding of the complex organization and function of cortex and its output circuitry, to identification of causes and therapeutic approaches for disease involving specific CFuPN subtypes, and to future strategies for cortical disease modeling, therapeutic screening, and regeneration/repair, since manipulation of these controls might also enable directed differentiation of pluripotent stem cells (m/h ES, iPS) for accurate disease models for mechanistic study and therapy discovery; directed differentiation of appropriate progenitors for regeneration; and potentially ?reprogramming? of some CFuPN (e.g., a subset of CThPN) into clinically desired subtypes (e.g., CSMN). We and others have made substantial progress identifying some broad, overall molecular/transcriptional controls over CFuPN subtype development, but much remains to be discovered at the subtype-specific level most relevant for disease and understanding of cortical function and organization. Building on our previous work, we propose to investigate novel regulatory biology of CFuPN subtype differentiation, diversity, and maintenance, substantially focused through a `lens' of regulation by Tle4 of identity acquisition of the distinct and clinically relevant CThPN, CSMN, and related SCPN subtypes (Aim 1), and to identify core transcriptional mechanisms at the molecular level that enable this precise regulation of CFuPN subtype diversity and balance (Aim 2). Further, based on highly motivating pilot studies indicating that Tle4 function is required to maintain CThPN vs. CSMN distinction even after circuit formation, we will rigorously and deeply investigate the potential to interconvert or ?reprogram? CFuPN subtype identity of some CThPN into ?genuine? CSMN by manipulation of Tle4 function at progressively later stages of development (Aim 3), exploring limits of subtype plasticity and circuitry manipulation.